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Full text of "The Cambridge Natural History"

RIBS

are there ribs with but one, the capitular, head. In the
posterior part of the series the two heads often gradually
coalesce, so that there conies to be but one, the capitular,
head. The "Whales also, at least the Whalebone Whales, are
exceptional in possessing but one head to the ribs, which
is the capitular. The first rib joins the sternum below, and
a variable number after this have the same attachment. There
are always a number of ribs, sometimes called floating ribs,
which have no sternal attachment. In the Whalebone Whales

FIG. 17.—A, First thoracic skeletal segment for comparison with B, fifth cervical vertebra
(Man), 6.v, Body of vertebra ; c, first thoracic rib ; e', cervical jib (which has be-
come united with the transverse process,, tr\ the two enclosing the cos to-transverse
foramen (f.c.t) ; stt sternum ; s&y, articular process of the arch (zygapophysis).
(From. Wiedersheim's Struckwre, qf M&n»)
it is the first rib alone which is so attached. As a rule,
to which the Whales mentioned are again an exception, the
rib is divided into at least two regions—the vertebral portion
which is always ossified, and the sternal moiety which is usually
cartilaginous. This is, however, often very short in the first rib.
They are, however, ossified in the Armadillos and in some other
animals. Between the vertebral and sternal portions an inter-
mediate tract is separated off and ossified in the Monotremata.
The ribs of existing mammals belong only to the dorsal region
of the vertebral column, but there are traces of lumbar ribs and
also of cervical ribs. In the Monotremata,, indeed, these latter